Many devices have been designed for the electrolytic dissociation of water to produce H.sub.2 and O.sub.2 gases, for example U.S. Pat. No. 4,465,570 Y. Oda et al., 1984; U.S. Pat. No. 4,726,888 M. Mc Cambridge, 1988. Some devices are designed such that a gas stream, U.S. Pat. No. 4,124,463 A. H. Blue, 1978, or a stream of inert liquid, U.S. Pat. No. 4,226,683 V. A. Vaseen, 1980, passes across the electrodes to dislodge the gases evolved from the water. However, there is no electrolytic device currently available which is designed to produce known H.sub.2 concentrations in a gas stream which purges the electrolytic cell. Such a device, if capable of producing low, stable concentrations of H.sub.2, would be extremely useful for the calibration of analytical instruments which measure low concentrations of H.sub.2 in a flowing gas stream. In order to produce stable H.sub.2 standard gases by electrolysis, the H.sub.2 evolved at the electrodes must be removed as it is formed, rather than accumulating in bubbles at the electrode surface. If bubbles are allowed to form, they produce irregularities in the H.sub.2 content of the gas in the air space above the electrolyte. The H.sub.2 generating device described in the present application minimizes bubble formation by reducing the surfaces on which bubbles may collect, and by passing a gas stream across the electrode surfaces during dissociation of water. In this way, H.sub.2 is swept away from the electrodes as soon as it is formed, and the accumulation of H.sub.2 in the air space above the electrolyte is determined only by the current supplied to the electrodes and the rate of gas flow through the air space.
Usual methods for calibrating instruments for the detection of low concentrations (of the order of a few parts per million) of gaseous H.sub.2 generally involve the use of pressurized gas cylinders containing H.sub.2 of known concentration, or the production of H.sub.2 standard gas mixtures using pressurized gases attached to gas mixing pumps. Both of these methods require adequate space for the storage and use of pressurized gases, and are also expensive in the long term. The H.sub.2 generating unit described herein requires little space and is inexpensive compared to accurate gas mixing devices or the cumulative cost of purchasing special H.sub.2 standard gases. An example of an instrument which could be calibrated using the electrolysis unit described in this patent application is a nitrogenase activity analysis system (NAAS) which measures nitrogenase activity as the rate of H.sub.2 evolution from N.sub.2 -fixing plant material. The NAAS is described in detail in copending patent application entitled An Apparatus for Measuring Low Concentrations of Gases in a Flowing Gas Stream, filed concurrently herewith and commonly assigned.